It is well known that the bandwidth of an optical communication link is limited by the noise margin required to ensure reliable communication. Typically, the noise margin is measured in terms of a signal-to-noise ratio at the receiver end of a link. In some cases, the optical signal-to-noise ratio (OSNR) is directly measured at the receiver. In other cases, signal parameters such as the eye opening, or bit error rate (BER) detected at the receiver are used as a proxy for the signal to noise ratio. In all cases, the noise margin can be allocated to four categories of phenomena:                Constant impairments, such as insertion losses and polarization coupling effects, which may vary between sites, or between individual pieces of equipment, but do not change with time;        Slowly Varying impairments, such as temperature effects, polarization phenomena in buried cable and aging, all of which have autocorrelation times of greater than one second;        Rapidly varying impairments, which are short-term transients having autocorrelation times of between one microsecond and one second. Typical examples of rapidly varying impairments include polarization mode dispersion due to above-ground cable movement, and optical power transients; and        Bursts with autocorrelation times of less than one microsecond.        
Typically, a noise margin of between 3 dB and 10 dB will be allocated to a link of the optical network, depending on the degree of reliability required and the specific unknown or varying parameters. This allocation is “static”, in the sense that it is selected based on the design of the link and its involved network equipment. In general, the allocated noise margin will be used in combination with forward error correction (FEC) to ensure that the link conveys subscriber traffic substantially without errors (e.g. BER≦10−15).
Within the optical network backbone, Synchronous Optical Network (SONET) Synchronous Transport Signalling (STS) and/or Synchronous Data Hierarchy (SDH) signalling is used extensively, because of its high bandwidth capacity and reliability. Within such synchronous networks, symbols are conveyed through each link at a fixed rate, irrespective of the actual subscriber traffic load at any instant. In order to maintain stability and synchronization across the network, any symbols that are not required for subscriber traffic (and control signalling) are encoded with spectrally white pseudorandom data.
As is well known in the art, subscriber traffic is highly variable, with daily, weekly and yearly patterns. For example, one-hour averages of night-time traffic may be only 10% that of mid-day traffic levels. During a given “peak busy hour”, the traffic load on large backbone links tends toward a Poisson distribution. As a result, the instantaneous traffic load within a link of the optical network will frequently be significantly lower than the (noise margin limited) link bandwidth.
The difference between the actual traffic load at any instant and the link bandwidth is frequently referred to as the link “headroom”. As the traffic load increases, the headroom decreases with an attendant rise in the risk of delayed or discarded packets, which is undesirable. Typically, for optical links in the network backbone, network operators provide a headroom of about a factor of four between the average “peak busy hour” traffic load and the link bandwidth. Clearly, the provisioning of such large amounts of headroom is expensive, because it requires that the network supplier lease significantly more link bandwidth than will actually be used, on average.
A technique that reduces network costs remains highly desirable.